Our invention comprises improvements in a four speed ratio automatic transmission of the kind disclosed in U.S. Pat. No. 4,347,765, which is assigned to the assignee of this invention. It is adapted especially for use in automotive vehicles having a forwardly mounted engine and rear drive wheels.
The transmission disclosed in the '765 patent comprises a hydrokinetic torque converter and a Ravagineaux type compound planetary gear unit disposed within a common transmission casing and arranged co-axially to accommodate torque transfer from the crankshaft of an internal combustion engine to a coaxial output shaft. The output shaft and the axis of the engine crankshaft are arranged in an inline relationship with the output shaft being connected drivably to the rear traction wheels of the vehicle through a driveshaft and a differential and axle assembly.
The transmission mechanism of the '765 patent includes an overdrive clutch that is connected through a central shaft to a crankshaft driven impeller of a hydrokinetic torque converter. The overdrive clutch serves also as a direct drive clutch and is engaged during third speed ratio operation as well as in a fourth overdrive speed ratio drive mode. Following operation in the third speed ratio, the overdrive brake is applied thereby providing a reaction point for the larger of two sun gears in the Ravagineaux type gearset. Thus the carrier which serves as the output member drives the driven shaft at an overdrive ratio of approximately 0.67:1. During operation in the third speed ratio, the torque delivery from the engine to the driven shaft of the transmission has a split torque delivery path wherein part of the torque is distributed mechanically through a central shaft and the balance of the torque is distributed hydrokinetically through the torque converter, the turbine of the torque converter being connected to torque input elements of the Ravagineaux the gearset.